1Field of the Invention
This invention relates to cardiac pacemakers and more particularly to rate responsive pacers which alter the pacing rate based on the patient's metabolic demand.
2. Description of the Prior Art
Recently rate responsive pacemakers have been introduced. These devices adapt the escape interval or pacing rate of the pacer to the metabolic demands of the patient. A variety of sensors and approaches have been employed to estimate metabolic demand including the measurement of blood oxygen saturation; blood pH; central core temperature; blood pressure and its derivatives as well as cardiac stroke volume.
One especially promising technique to estimate metabolic demand involves the measurement of the patient's breathing rate and the computation of the mass flow rate of air into (and out of) the patient's lungs. This latter quantity is called the minute ventilation.
The use of impedance plethysmography to generate this type of information is known from U.S. Pat. No. 4,596,251 to Plicchi and Canducci. This prior art pacer derives the respiration rate through a thoracic impedance measurement using an auxillary lead.
Other investigators have proposed other sensor techniques and locations. Funke reported on the use of a microphone to monitor intrapleural or intrathoracic pressure in "Ein Herzschrittmacher mit Belastungsabhangiger Frequenzregulation" published in 1975 in BioMedische Tecknik. In this article Funke also suggested, and rejected, a system based on the detection of electrical activity of the diaphragm or the intercostal musculature.
More recently, Cunningham et al have demonstrated the electronic integration of the diaphragm electromyogram to generate a control signal proportional to respiratory minute volume for use as the controlling physiological input for a pacemaker; see "Variation of Cardiac Pacemaker Rate Relative to Respiration", IEEE, 1979.
Other methods of deriving a respiratory signal have been proposed more recently including the sensing of amplitude variations of the heart action signal as seen from EPO patent application No. 0 226 164 published June 24, 1987. It has also been proposed to extract an aspiration signal from respiration modulation of the impedance of the vascular bed as shown in U.S. Pat. No. 4,702,253 to Nappholz. Or, such a signal may be derived from the pacing current and voltage in a pacer as shown in U.S. Pat. No. 4,694,830 to Leckholm.
In each of these prior art devices, a respiration dependent signal is extracted and used to drive the pacing rate. The present invention is like the prior art in this regard.